Secret wired radio transmission system



Aug. 9, 1932. AS ELL 1,870,490

SECRET WIRED RADIO TRANSMISSION SYSTEM Filed Oct. 20, 1930 2 Sheets-Sheet 2 gt M Q I Q l .m I I k g I i 2$, I I l k I L 0 f5 'fz fi'e uency k-$ .33 s Q Q I we Frzquency INVENTOR Joseph L. Cassell.

BY #QRNEY Patented Aug. 9, 1932 UNITED STATES PATENT OFFICE JOSEPH It. CASSELL, OF NEW YORK, N. Y., ASSIGNOB T WEED RADIO, INC 01' NEW YORK, N. Y., A. CORPORATION OF DELAWARE SECRET 'WIRED RADIO TRANSMISSION SYSTEM Application filed October 20, 1980. Serial No. 489,772.

My invention pertains in general to systems for transmission of high frequency signals and specifically relates to a wired radio a system for transmission and reception of high frequency signals in which means are provided to prevent theft of signals by persons other than those for whom the signals were originally intended.

In the system of wired radio broadcasting in which modulated high frequency currents are impressed upon commercial light and power distribution networks, the light and power lines. extend to many consumers who subscribe only to the light and power service 1 and not to the wired radio service. It is therefore desirable that means he provided to prevent the intelligible reception of wired radio signals by consumers other than those who are authorized to receive such s'i als and who have proper wired radio receiving apparatus.

One .of the objects of my invention consists in providing a system for the transmis- *sion and reception of high frequency signals in which meansare provided for transmittin an intelligible audible frequencyband an an unintelligible, or noise modulated, frequency band and in which receiving means are provided which are responsive only to the intelligible frequency band. Another object of my invention consists in producing a secret wired radio transmission system in which an intelligible and an unintelligible frequency band are transmitted over a commercial light and power distribution network and in which means are provided which are adapted to receive only the intelligible frequency band.

Another object comprises producing. a secret wired radio transmission system having transmitting means adapted to intelligibly modulate a definite portion of a frequency spectrum while unintelligibly modulating another portion of the frequency spectrum, and recelving means responsive only to the intelligibly modulated portion of the frequency spectrum.

A further object consists in providing a secret wired radio transmission system in which intelligible modulating currents and unintelligible modulating currents are transmitted by a single carrier current and in which receiving means are provided which are responsive only to the intelligible modulatin g currents transmitted by the carrier.

I accomplish the above desirable objects in a novel system for transmission and reception of wired radio signals having means for transmitting intelligible and unintelligible audible frequencies by means of a single carrier current, and receiving means responsive only to the intelligible audible frequencies transmitted by the carrier current.

In the drawings which accompany and form a part of the specification and in which like reference numerals designate corresponding parts throughout:

Fig. 1 is a diagrammatic representation of one embodiment of the secret wired radio transmission system of my invention;

Fig. 2 is an elevation of apparatus used in the embodiment of Fig. 1 for varying the pelgative biasing potential of a thermionic Fig. 3 is a graphical representation of the transmission response of the apparatus used in my invention; and

Fig. 4 is another graphical representation of the transmission response of the apparatus used in my invention.

I Referring to the drawings in detail, and particularly to Fig. 1, an oscillator 1 com- PIISBS a source of high frequency oscillations which are directed to a power amplifier 2. A modulator 3 under control of a microphone 4 directs modulating currents to the power amplifier 2 to modulate the oscillations, or carrier current from the. oscillator 1 in accordance with intelligible sound waves,

such as speech and music, picked up by the microphone 4. The output of the power amplifier 2 is fed to the bandass filter stems Sand .6. The band-pass ters 5 an 6 here shown, are of the inductively coupled mesh ty e. However, other types of filters employ" g different networks to accomplish the desired results may be used, if preferred, witho ut departing from the scope of my invention. I

' Fig. 3 is a graphical representation of the combined transmission response of filters 5 and 6, the 'cut-ofl' frequencies for filter 5 being represented b 72 and f while the cutofi frequencies for lter 6 are represented by 7''} and i The filter 5 is, therefore, responsive to, and will pass only, frequencies within the-band represented by f and the filter 6' is responsive to, and will pass only, frequencies within theband represented by f --f In Figs. 3 and 4 the carrier freuency is represented by C at a point on t e frequency spectrum within the unattenuated frequency band adjacent the lower cutofi frequency f As contemplated by my invention, the audible frequencies under control of microphone 4 are restricted to the frequency band f while the frequency band -f is employed to transmit unintelligi le sounds which will guard the adjacent fre uency band f f and prevent its reception y other than the proper receiving apparatus.

' Referring again to Fig. 1, it will be seen that the output of the filter 6 is directed to the grid of the thermionic tube 7 and to a variable resistor 8, as shown. The resistor 8 is in series with a battery 9 which serves as a source of negative biasing potential for the grid of the tube 7. The battery 9 is connected to the negative side of the filament of the tube 7. Battery 10 is provided as a source of filament supply for the tube 7 A battery 11 is connected through inductance 12 to plate of the tube 7 and serves as a source of plate potential. The inductance 12 forms plant of another filter 13 which is similar to ter 6 and is reponsive to frequencies within the band f 3 (see Fig. 3) This filter 13 is provided on the output side of the tube 7 to attenuate harmonic frequencies developed by the tube and to insure that the output is within the desired frequency range.

The output oflfilters 5 and 6 are connected, respectively; through capacitive couplings 14:

and 16 to power lines 17 The power lines 17 extend from a source of commercial light and power current 18 and are indicative of a normal light and power distribution network. The capacitive couplings 1 1 and 16 allow the passage of high frequency currents to the lines 17 but prevent the commercial power, which is of low frequency, from feeding back into the high frequency filter systems.

A receiving filter system 19 is connected through capacitive couplings 20 to the power lines 17 The filter system 19, here shown,

- is of the inductively coupled mesh type.-

However, as before stated, other types of filter networks may be employed. The filter 19 has a transmission response graphically represented in Fig. 4, the upper cut-off frequency being represented by i the lower cutoff frequency being represented by f It is evident, then, that the filter 19 is responsive.

to, and will pass only, frequencies iii the newest band f and the frequencies within the band f f as well as all others, will be rejected. The filter 19, therefore, is responsive only to frequencies passed by the: transmitter filter 5 represented by the frequency band fig-f1 in Fig. 3. The output of the filter 19 is fed to a receiver 21 having a normal detector and amplifier system adapted to reproduce the acoustical vibrations picked up by the transmitter microphone {1.

Fig. 2 illustrates the construction of mechanlc'al parts associated with the variable,

25, composed of insulating material, such as fibre, is reciprocatively positioned on the guide rod 24. A contact spring 26 is mount; ed on the rider 25 so as to slidably engage the resistance wire of the resistor 8. A motor 28 is mounted on the base 27 and has a crank-disk 29 mounted upon one end of its armature shaftf30. A. connecting rod 31 is pivot-ally connected at one end to the rider 25 and at the other end to an eccentric point of the disk 29. Rotation of the armature shaft 30 causes a corresponding rotation of the disk 29 which imparts a reciprocating movement to the rider 25 thereby periodically varyingthe eifective resistance value of the resis or 8. v

The operation of the foregoing is as follows: y Tntelligible sound waves impinged upon the microphone & act through the modulator 3 to modulate the high frequency oscillaoscillator 1-. ,This modulated carrier current is properly amplified by the amplifier 2 andtlons; or carriercurrent, produced by the the frequency band -f ,-as shown in Fig.

3. The filter 6 rejects all frequenciesiexcept those within a'frequency band f shown in Fig. 8 The output of the filter 5 is fed through capacitive couplings 14 to the power lines 17, while the output of the filter system 6 is fed into the tube-,7. The value of the grid biasing potentiaP of the tube 7 fed through the resistor 8 is altered at audible frequencies by varying the effective value of the resistor 8. This is accomplished by causing contact spring 26 to move back and forth along the resistance wire of the resistor 8., This periodic increase in grid biasing potential serves to periodically block the tube Zby increasing the negative charge on the grid thereof. Such action produces an unintelligible, or noise, modulation of the signals from filter 6 and which is of an audible frequency dependent upon the speed of rotation of motor 28. The output of the tube .7 is then fed through another filter 13 similar to filter 6 and thence through capacitive couplings 16 to the power lines 17. The filter 13 insures restriction of the frequencies to within the band f f of Figs. 3 and 4. The total, or combined, filter transmission output therefore extends over the band f f in Fig. 3, in which the band f f is intelligible while the band f f is unintelligible.

The receiving filter 19 and the receiver 21.

receive the intelligible signals lying within the band f f Other receiving apparatus, not having filter systems of the same characteristics as filter system 19, have a comparatively broader response, such as graphically represented by the curve 1- in Fig. 4. His. evident that such a receiver would not be able to distinguish between the frequency bands f f and f f The resultant reception is therefore rendered unintelligible by the presence of the unintelligible, or noise modulated, frequencies within the band f f By such a system, then, it is possible to maintain secret distribution of signals over commercial light and power distribution lines to only those authorized subscribers who are equipped with proper receiving apparatus having filters of predetermined characteristics.

Although I have shown a preferred embodiment of my secret wired radio transmission system, it is, of course, to be understood that changes can be made therein without departing from the intended scope of my invention. I do not, therefore, desire to limit myself to the foregoing except insofar as may be pointed out in he appended claims.

What I claim as new and original and desire to secure by Letters Patent of the United States is:

1. A transmission system comprising, in combination, a source of carrier current, means adapted to modulate said carrier current at audible frequencies, a filter system responsive to high frequency current within a predetermined frequency band, a second filter system responsive to high frequency current within a predetermined frequency band different from said first-mentioned frequency band, a thermionic tube having grid and.

plate electrodes, a connection between the output of said second filter and the grid of said thermionic tube,a source of biasing otential for said grid, means adapted to periodically varv the value of said biasin potential, a third filter responsive to high equency current within the frequency band to which said second filter is responsive, a connection between the plate of said thermionic tube and said third filter system, and receiving means first-mentioned filter is responsive.

2. A transmission system comprising, a

source of carrier current, means adapted to modulate said carrier current at audible frequencies, a filter system responsive to high frequency current within a predetermined frequency band, a second filter system responsive to high frequency current within a predetermined frequency band different from said first-mentioned frequency band, a third filter in series with said second filter and responsive to high frequency current within the same frequency band to which said second filter is responsive, means interposed between saidsecond and said third filter systems and adapted to unintelligibly, or noise, modulate the high frequency current passed by said first-mentioned filter, and receiving means responsive only to high frequency current within the frequency band to which said firstmentioned filter is responsive. 3. A transmission system in accordance with claim 2, but in which wire lines are provided from said first and third filters to said receiving means. 

